Field of the Invention
The present invention relates in general to a method of joining heat-treatable aluminum alloy members by friction stir wielding and a joined product for press forming obtained by the method. More particularly, the invention relates to such a method capable of advantageously providing a joined product with good ductility, accordingly with good press formability while effectively preventing occurrence of fracture at a stir zone and heat-affected zones of the joined product obtained as a result of joining the heat-treatable aluminum alloy members by friction stir welding, and such a joined product for press forming advantageously produced by the method.
Discussion of Related Art
Conventionally, a plate or sheet member made of a heat-treatable aluminum alloy is used as a blank for press forming. Such a blank is subjected to the press forming, thereby producing various kinds of press products. In a press working operation of the heat-treatable aluminum alloy plate member, rather than a T6-treated aluminum alloy plate member (i.e., aluminum alloy plate member with T6 temper) which assures high strength, a T4-treated aluminum alloy plate member (i.e., aluminum alloy plate member with T4 temper) is preferably used because of its excellent ductility and softness lower than the T6-treated aluminum alloy plate member.
In the meantime, in recent years, a plurality of aluminum alloy members are joined together to provide a joined product, and the obtained joined product is subjected to a press working operation, a bending operation and the like for improving a material yield as a result of elimination of cutting and trimming after the press working operation and simplifying the production process. More specifically described, a plurality of aluminum alloy members are butted and welded together into the integrally joined product, thereby providing one blank for press forming, i.e., a so-called tailored blank. A technique of performing suitable press forming on the thus obtained blank is widely employed in the field of a press product for an interior panel of automotive vehicles, for instance.
In addition to the improvement in the material yield and the simplification of the production process, such a press forming technique provides an advantage of utilizing, as a material for the blank, a small-sized metal plate which cannot be used in an ordinary press forming technique. Further, where a blank obtained by joining together the heat-treatable aluminum alloy members having mutually different thickness is used as a blank for press forming, it is easy to produce a press product having required strength at required portions thereof.
As a method of joining the heat-treatable aluminum alloy members for obtaining such a blank for press forming, various welding methods are conventionally employed such as TIG welding (tungsten inert-gas welding), MIG welding (metal inert-gas welding), laser welding, and friction stir welding. Among those, an increasing attention has been drawn to the friction stir welding process which is a sort of solid-phase welding or joining process involving a relatively small amount of heat generation, relatively small degrees of softening and distortion, as compared with a fusion welding process, and therefore assuring a sufficiently high degree of welding strength or joint strength. However, even where the T4-treated members are butted and joined together according to such a friction stir welding process to provide the blank, softening inevitably occurs at heat-affected zones of the blank which are located adjacent to a stir zone at which the T4-treated members are welded and which are affected by a heat generated in the friction stir welding process. If such a blank is used for press forming in the as-welded condition without undergoing any post-treatment, stress concentrates locally on the heat-affected zones at which the strength (hardness) is the lowest due to the heat generated upon welding, so that the heat-affected zones tend to deform faster than the other portion of the blank and often break or fracture. The fracture or breakage at the heat-affected zones raises a problem of limitation in a configuration of the press product to be obtained from the blank and a degree of elongation or ductility (a deformation amount) of the blank.
To deal with the problem indicated above, there have been made: (1) proposals to improve a joint strength of a welded region by performing an age-hardening treatment after the heat-treatable aluminum alloy members have been welded together (as disclosed in the following Patent Documents 1-5); and (2) proposals to improve an overall joint strength of a joined portion including the heat-affected zones by performing an age-hardening treatment after the heat-treatable alloy members have been are joined by friction stir welding (as disclosed in the following Patent Documents 6-8).
The proposals (1) will be explained in detail. Patent Document 1 (JP-A-5-117826) discloses a method of producing a rim made of a high-strength aluminum alloy comprising: forming an aluminum alloy member into a rim shape after performing flush butt welding; and heating the rim-shaped member at a temperature of 170-200° C. Patent Document 2 (JP-A-8-246116) discloses a method of producing an aluminum-alloy made wheel with increased strength comprising: welding a disc made of an Al—Mg—Si series alloy to a rim made of an aluminum alloy; cooling the welded region at a rate of not less than 1° C./second and subsequently heating at a temperature of 100-200° C. for 5-60 minutes. Patent Document 3 (JP-A-9-177974) discloses a technique of restoring a heat-softening phenomenon by a welding heat by performing an aging treatment which involves heating at a temperature of 150-200° C. for not less than 30 minutes after welding. Patent Document 4 (JP-A-11-199994) discloses that a welded member obtained by welding Al—Mg—Si series alloy extruded members is subjected, after quenching, to an aging treatment at a temperature of 150-220° C. for 3-24 hours, for increasing the strength. Patent Document 5 (JP-A-2002-294381) discloses a welded joint of an excess Si-content type JIS-A 6000 series aluminum alloy obtained by welding aluminum alloy members and having joint strength and joint elongation restored by performing an aging treatment at a temperature of not higher than 180° C. for about 10-50 minutes.
The proposals (2) will be explained in detail. Patent Document 6 (JP-A-11-104860) discloses a welded joint with intended strength obtained by: joining together members by friction stir welding; subjecting, immediately after the welding, a welded portion to forced air cooling for placing the welded portion into a hardened state; and performing a suitable artificial age-hardening treatment or natural age-hardening treatment. Patent Document 7 (JP-A-2000-61663) discloses a method of joining aluminum alloy members with T1 temper comprising: joining the members by friction stir welding such that a time period during which heat-affected zones located outwardly of a stirred region are heated at a temperature of not lower than 300° C. is controlled to be within one minute; and performing an aging treatment at a temperature lower than 300° C. for about from 10 minutes to 24 hours, thereby attaining the joint strength that is not less than 95% with respect to yield strength of a base material. Patent Document 8 (JP-A-2002-346770) discloses that aluminum-based alloys whose average crystal grain size is 100-5×103 nm are joined together by friction stir welding and then subjected to an aging treatment at a temperature of 100-200° C.
To assure a high degree of joint strength, Patent Document 9 (JP-A-2001-321948) discloses a method of welding heat-treatable aluminum alloy members comprising a step of performing, prior to a welding process such as TIG welding, MIG welding, laser welding and friction stir welding, a sub-aging treatment in which an aging treatment is performed on the aluminum alloy members at a temperature lower than and for a time period shorter than those in aging treatment conditions in which maximum strength is obtained. Patent Document 9 teaches, in its example, a heat treatment carried out at a furnace temperature of 160-200° C. for three hours as the specific conditions of such a sub-aging treatment.
By performing the age-hardening treatment after fusion welding or friction stir welding or by performing the sub-aging treatment before welding as disclosed in the above-indicated Patent Documents, the joint strength is improved. Those Patent Documents, however, are silent about press formability and ductility of the joined product obtained by joining the aluminum alloy members. Accordingly, the strength of the heat-affected zones is lower than those of the welded region and the base material, so that the stress concentrates on the heat-affected zones, causing fracture at the heat-affected zones. Therefore, there still exists the problem of limitation in the configuration of the press product to be obtained from the joined product and the elongation degree (the formable amount) of the joined product.
Further, Patent Document 10 (JP-A-10-225781) discloses a friction stir welding process in which, prior to friction stir welding, a portion ahead of a probe as seen in its moving direction is heated at a temperature of not higher than 500° C., preferably in a range of 100-300° C. using an external heat source such as a laser radiation, a gas flame or a heater. According to the disclosed method, the members are joined without suffering from any failure while improving a rate of joining the members and increasing a service life of the probe. Patent Documents 11 and 12 (JP-A-2003-80381 and JP-A-2003-94175) disclose a technique of heating portions ahead of and behind a joint region at a temperature in a rage of 100-300° C. by means of a welding torch or induction heating for shortening a time before initiation of friction stir welding or for preventing occurrence of cracking at the joined region including the heat-affected zones. In the methods disclosed in those Patent Documents 10-12, although the heat treatment is performed on the aluminum alloy members prior to the friction stir welding, the heat treatment is performed only for reducing resistance to deformation and shortening the time before initiation of friction stir welding. None of these methods aim at refining the aluminum alloy members.